Gas burner for premixed combustion

ABSTRACT

A gas burner for premixed combustion is positioned in a combustion chamber and comprises two components, a first defined by a distributor for a gas and air mixture and the second defining the burner shell. The components being close together but spaced apart. The distributor has a perforated distribution surface through which the gas-air mixture passes directed towards the burner shell, which also has a perforated burning surface on which the flame caused by the combustion of said mixture is generated, to create a thermal load within the combustion chamber. The thermal load in this latter is disuniformly distributed.

The present invention relates to a gas burner for premixed combustion, in accordance with the introduction to the main claim.

It is known that such a gas burner (for a boiler) can be of cylindrical or flat form, and be of steel or of fibre composite (or other known suitable material). This burner receives a gas-air mixture, obtained externally to it in known manner and fed internally to one end of it through known pipes; this mixture is activated by an ignition electrode located in a suitable position relative to the burner such as to generate a flame on the outside of the burner, or on the other end from that at which said mixture arrives. The burner is usually positioned in a combustion chamber from which the flue gases generated within it are removed through an appropriate exit and directed to discharge. p Such a premixed combustion burner is known to comprise a first component is known commonly as a “distributor” and a second component defining the (outer) shell of the burner. The distributor receives the air-gas mixture and presents a perforated distribution surface through which this mixture passes. The distributor is positioned in proximity to the burner shell formed by a perforated “burning” surface from which the air-gas mixture emerges and on which the flame is generated by activation of the ignition electrode. If the burner is of cylindrical form, for example, these components are cylindrical and coaxial, the distributor being contained within the burner shell and receiving said mixture internally thereto.

The burning surface is known to consist of a perforated surface (with holes or slits of known dimensions), for example of steel or of a steel fibre mesh or other suitable material.

The distribution surface consists normally of a surface, for example of steel, suitably perforated such as to adequately distribute the mixture over the entire burning surface and generate the flame over its total area so as to equally distribute the burner thermal load uniformly over this latter (W/cm² of burning surface).

A solution is also known in which the burning surface indeed generates a thermal load distributed over its entire area (with the flame present on the entire surface), although in a particular point or zone of this latter this load varies relative to the average load (normally that zone within the interior of the flame sensing electrode); the reason for this is to obtain in these points or zones a higher flame signal which enables better flame sensing and/or better combustion control compared with known solutions. However, this solution limits the working power range and does not allow optimal burner control as the flame signal by which this control is carried out varies substantially, for example on varying the family gas type used in the burner.

The fact of generating the flame over the entire burner surface and the positioning of this burner in a position along the central axis of the combustion chamber involves the presence therein of a thermal load equally distributed within the chamber volume about the burner.

This solution, however, presents various problems. For example, a real consequence is that the burner surface temperature is dishomogeneous: in this respect, that zone corresponding with the off-gas exit from the combustion chamber is normally substantially colder than that zone opposite to this exit. Moreover those zones involved in the movement of the off-gases originating from the zone opposite the off-gas exit and those transiting on the burner sides are hotter than that zone opposite the exit. The result is a disuniform heat exchange with the usual heat transfer fluid circulating through the boiler combustion chamber and imperfect heating of the burner surface, so influencing boiler performance.

In addition, a substantial noise can be generated (in the form of a “bang”) during ignition. To this can be added that, mainly but not only in those cases of combustion control linked to the flame signal used as feedback in the combustion process, correlation between the flame signal and combustion on the basis of the power range in which the burner operates is not optimal.

Finally, noises (generally whistles) can also be generated during combustion under normal working conditions and/or as climatic conditions vary.

An object of the present invention is o provide an improved burner by which the drawbacks of known burners are overcome.

A particular object of the invention is to provide a burner the thermal load of which is “tuned” on the basis of the form and volumes of the combustion chamber.

Another object is to provide a burner of the stated type the surface is temperatures of which are on the average more homogeneous.

A further object is to provide a burner with lower CO and nitrogen oxide (NOx) emissions than known solutions.

Another object is to provide a burner which is of low noise (when in application, i.e. during use) in comparison with known burners, and a burner which enables better combustion control.

These and other objects which will be apparent to the expert of the art are attained by a burner in accordance with the accompanying claims.

The present invention will be better understood from the accompanying drawings, which are provided by way of non-limiting example and in which:

FIG. 1 is an exploded perspective view of a burner according to the invention;

FIG. 2 is a side view of the burner of FIG. 1;

FIG. 3 is a section on the line 3-3 of FIG. 2;

FIG. 4 is a section on the line 4-4 of FIG. 2;

FIG. 5 is a section on the line 5-5 of FIG. 2; and

FIG. 6 is a schematic cross-section through a burner of the invention within a boiler combustion chamber.

With reference to said figures, a burner according to the invention is indicated overall by 100 and comprises a first component defined by a distributor 1 for a gas-air mixture (obtained before being fed through a known conduit, not shown, to the distributor), and a second component (close to but separated from the first 1) defined by the burner outer shell 2. This distributor 1 and shell 2 are of tubular cylindrical form in the example shown in the figures, but could also be of flat or other form. In the example, the distributor 1 is internal and coaxial to the shell 2. End closure members 17, of which one is flanged, close the ends of the burner 100.

The distributor 1 presents a distribution surface 3 which is partially perforated, i.e. in which zones 4 (central in the figures) and 4 a (lateral in the figures) are included provided with through holes 5 plus zones 6 which are completely smooth and solid, i.e. not perforated. The holes 5 connect a space or interspace 7 present between the distributor 1 and the shell 2 and facing an external or outer side 10 of the distributor to an internal part 11 of this latter (i.e. to a compartment facing the inner side 13 of the distributor opposing the outer side 10).

In this manner, the mixture which reaches the inner part 11 can be transferred to the interspace 7 through the holes 5. Because of the disuniform position of these holes in the surface 3, this transfer takes place in differential manner within the interspace 7, with different mixture quantities reaching the burner shell 2. There is hence dishomogeneous feed of gas-air mixture to the shell 2. This latter comprises a burning surface 15 having an inner side 16 facing the interspace 7 and an outer side 18 on which the flame forms (obtained by “igniting” the mixture by means of a known ignition electrode, not shown). The burning surface 15 presents a plurality of through slits and holes 20 which connect the interspace 7 to the outer side 18 of the shell 2 and enable the mixture to pass onto this latter to form the flame. These slits and holes 20 define a flame generation zone 21 which is localized in the burning surface 15 and is only partially superimposed on the zone 4 of the distributor 1. This burning surface 20 lies to the side of and/or alternates (as in the figures) with zones 22 without holes and by which no flame is generated.

By virtue of the particular shape of the distributor surface 3 defined also on the basis of the burning surface 15, the burner 100 enables a deliberately non-uniform thermal load to be generated in which there is a disuniform outflow of mixture onto the burning surface 15 and hence disuniform flame generation on it.

According to the embodiment of the figures, the burner thermal load is greater where the zones 4 and 4 a of the distributor face each other in each flame generation zone 21, whereas it is less (than the overall average load) where these zones are not mutually superimposed and/or involve a mixture quantity distributed only by zones (such as the lateral zones 4 a of the example of the figures) of the distributor 3 facing the zones 22 of the burner shell 2.

The thermal load modification and disuniformity can be further highlighted and achieved, not only by eliminating holes from each surface portion of the distributor, but also by modifying the diameter and pitch between the distributor holes 5 and/or the position and size of the slits and holes in the burning surface 20 or the mutual arrangement between the distributor holes 5 and the holes of the burner shell 2, hence in this manner modifying the distribution of the mixture quantity leaving the surfaces 3 and 15. Alternatively or in addition thereto, the thermal load within a usual combustion chamber 50 in which the burner 100 is positioned can be modified by disposing this latter in an eccentric position contrary to that usually done where the burner 100, as in FIG. 6, is positioned along the longitudinal central axis of that chamber.

Hence by virtue of the invention, the thermal load of the burner 100 can be adapted on the basis of its own geometry, on that of the combustion chamber 50 and on the evacuation path for the off-gases from it (via a discharge 60). This enables various advantages to be obtained. p For example, a more uniformly distributed cooling of the burner 100 throughout the burning surface 15 can be achieved by increasing the thermal load in the normally cooler zones (for equal working power) and decreasing it in the normally hotter zones, again for equal conditions. This enables surface temperatures to be achieved which are on the average more homogeneous independently of the fact that a particular surface portion is grazed by greater is heat flows than other portions which are adjacent or positioned in front of the off-gas discharge.

Moreover, by suitably choosing a zone 21 of the burner shell 2 with a particular thermal load (for example by modifying the mixture exit velocity by varying the diameter or position of the distributor holes 5 or of the shape and arrangement of the holes and slits in the burning surface 20), the noise can be improved (by decreasing it) during burner ignition and flame propagation.

In addition, significant CO and nitrogen oxide (NO_(x)) emission reductions can be achieved by avoiding locating the holes 5 of the distributor 1 along the opposing edges 2A and 2B defining that burner zone with the perforated burning surface 2. If this is not done, then particularly high combustion temperatures and a particularly high surface temperature are generated in this zone such as to negatively influence the aforestated parameters.

By virtue of the invention, a low combustion noise can also be achieved at steady working and under the different environmental conditions, even as the working CO2 varies. This enables greater working flexibility of the burner: for example it can favour the use of electronic combustion control systems which normally execute self-verification functions, and burner auto-calibration by temporarily setting high CO2 values which with known burner solutions could give rise to noise and whistle generation.

Because of the fact that zones of the burner shell 2 exist generating variable thermal loads, it is possible to locate a usual flame sensing electrode (or equivalent element) in a suitable zone of the burner (for example positioned such as to pass through zones of different thermal load) in a zone such as to improve correlation between the sensed flame signal and combustion at different working powers and with varying family gas type, for example in changing from 2nd family gas (methane) to 3rd family gas (LPG). In this respect, it is known that in monitoring the flame, a flame signal is obtained which is a function of the power and combustion, this signal normally having different dynamics between the two families. By means of the invention, these signals for the two gas families approach each other to such a point that, even if there is a setting error for the gas type used (for example, the burner is set to operate on methane, but is fed with LPG) it remains in an environment or range of safe operation. This enables burners with relative control systems to be constructed which can operate with gases of different families, without having to proceed to modify internal components of the application (for example the boiler). By developing specific functions in the control algorithm for these systems, it also enables identification of the gas family with which the burner is fed, so enabling the working parameters of this latter to be semi-automatically or automatically adapted to the gas family sensed at the burner entry.

One of these functions can be for example that of verifying, in particular stages of operation, the dynamics of the flame signal (substantially different from one gas family to another) as the working parameter varies, for example as the gas flow varies for fixed air flow or, vice versa, as the air volume varies for fixed gas flow.

Finally, if the gas type has been incorrectly set during installation, the correlation between the flame signal and the working lambda coefficient, i.e. the coefficient which defines the ratio between the air and gas, means that if the gas family changes, the invention is able to maintain CO values below the legal limits and CO2 values close to the optimal working value, so guaranteeing operational safety of the application.

Various embodiments of the invention have been described. Still others are possible in the light of the aforegoing, said further variants being considered as falling within the scope of the invention as defined by the following claims. 

1. A gas burner for premixed combustion positioned in a combustion chamber and comprising two components, a first component defined by a distributor for a gas and air mixture and a second component defining the burner shell, said components being close together but spaced apart, the distributor having a perforated distribution surface through which the gas-air mixture passes directed towards the burner shell, which also has a perforated burning surface on which the flame caused by the combustion of said mixture is generated, to create a thermal load within the combustion chamber, wherein the thermal load in this combustion chamber is disuniformly distributed.
 2. A burner as claimed in claim 1, by comprising dishomogeneous distribution of the air-gas mixture towards the burner shell, said dishomogeneous mixture distribution generating a disuniform distribution of the thermal load within the combustion chamber.
 3. A burner as claimed in claim 2, wherein the distributor has its distribution surface at least partially perforated, mixture distribution being rendered non-homogeneous over said surface by through holes present therein at non-uniform pitch and/or by differentiation of the diameter of the holes.
 4. A burner as claimed in claim 3, wherein the distributor has its distribution surface partially perforated and presenting at least one zone provided with through holes and at least one non-perforated zone, said conformation enabling dishomogeneous distribution of the gas-air mixture.
 5. A burner as claimed in claim 1, wherein the burning surface presents at least one flame generation zone provided with through holes and/or slits and by which the flame is generated, and at least one zone not provided with holes and without flame generation.
 6. A burner as claimed in claim 3, wherein the perforated zone of the distributor only partially corresponds to the flame generation zone of the burner shell.
 7. A burner as claimed in claim 1, wherein at least part of the holes of the distributor and/or at least part of the holes and/or slits of the burner shell present mutually different sections.
 8. A burner as claimed in claim 1, wherein at least part of the holes of the distributor and/or at least part of the holes and/or slits of the burner shell are at distances apart which are different from the distances between other holes of said distributor and burner shell.
 9. A burner as claimed in claim 1, wherein those zones of the distributor provided with the holes are spaced from or not present in the opposing free edges which bound the perforated burning surface of the burner shell.
 10. A burner as claimed in claim 1, comprising at least one of the following characteristics: the distributor and the burner shell are of tubular cylindrical shape and are mutually coaxial, an interspace being present between said components, the burner is located in a position eccentric to a longitudinal central axis within the combustion chamber, the dishomogeneous distribution of the thermal load is a function of the dimension and/or form of the combustion chamber.
 11. A burner as claimed in claim 4, wherein the perforated zone of the distributor only partially corresponds to the flame generation zone of the burner shell.
 12. A burner as claimed in claim 5, wherein the perforated zone of the distributor only partially corresponds to the flame generation zone of the burner shell.
 13. A burner as claimed in claim 1, wherein those zones of the distributor provided with the holes are not present in the opposing free edges which bound the perforated burning surface of the burner shell. 